I try to install mac os cause I didn't know what I was doing when I installed Ubuntu... I want mac os back but everytime i insert the install dvd and hold option or c all i get is a blinking folder looking back at me. What do I do?
The Ubuntu installer probably reformatted the whole drive using ext2 or ext3, whereas current versions of Mac OS X will only install on HFS+. To fix this, boot up with the Mac OS X install CD, then run Disk Utility from the Utilities menu. Disk Utility will let you check to make sure the drive has at least one HFS+ volume on it, and if it doesn't, create one.
If you get nothing at all, there are two good articles on Apple's support site to check out and follow through on, they are fairly thorough.
I know it says gray screen, not a black screen, but it applies Mac OS X: Gray screen appears during startup http://support.apple.com/kb/TS2570
Resolve startup issues and perform disk maintenance with Disk Utility and fsck http://support.apple.com/kb/ts1417
Which you will probably want to see if you can still boot into single user mode to do via Command+S on boot, http://support.apple.com/kb/HT1492
If you cant boot into single user mode and you cant boot to your disc, and you cant ssh into it, then see if you can boot into target disk mode http://support.apple.com/kb/ht1661. If you cant do ANY of that, then I would say you might want to wipe it with a different live cd and try again.
There are only a few possiblities that I can see:
You aren't holding down the Option key or C key early enough in the sequence. Make sure you're holding it down immediately following the the startup chime.
Your keyboard isn't work, so it doesn't matter when you hold down the key. Try another one.
Your CD/DVD drive is no longer working properly.
Installing Ubuntu somehow messed with the parameter RAM. Hold down Cmd+Opt+P+R immediately after the chime until the chime repeats.